The ACC receptor family appear
The ACC receptor family appear to be attractive targets for the development of novel anthelmintics. With this in mind we also tested the activity of the anthelmintics levamisole and pyrantel which activate the nematode nAChR (Martin and Robertson, 2007). We found both molecules were partial agonists for the Hco-ACC-2 receptor. However, we were surprised to observe that the mutations we introduced into the receptor minimally effected in their activity. This may highlight the distinct manner at which these anthelmintics bind to the ACC receptors. Molecular models for docked pyrantel and levamisole place both molecules within 5 Å of W248. However, the overall positioning of levamisole and pyrantel within the ACC-2 binding site are clearly different (Fig. 5). How this relates to the differences in efficacy we observed between the two molecules requires further examination.
This study has described the isolation of a member of the ACC family from H. contortus and examined two residues in the binding site that are important for agonist recognition. Further research on the essential requirements for agonist recognition of ACC receptor can provide insight into the GW441756 of cholinergic neurotransmission and may possibly lead to the discovery of novel cholinergic anthelmintics.
Declarations of interest
Acknowledgements This research was funded by grants from Natural Sciences and Engineering Research Council of Canada (Grant #210290) and the Canadian Foundation for Innovation to SGF. The authors declare no conflicts of interest.
Introduction Rhipicephalus microplus (Canestrini, 1888) parasitism is one of the major limitations of cattle production in countries with subtropical and tropical climates. Resistance is a problem associated with the chemical control and it has been described against all chemical families of acaricides (e.g. organophosphates, synthetic pyrethroids, amidines, macrocyclic lactones, phenylpyrazoles and phenylureas). The existence of multiple acaricide-resistant tick strains has been frequently reported worldwide (Fernández-Salas et al., 2012; Muyobela et al., 2015; Rodríguez-Hidalgo et al., 2017; Fular et al., 2018) and recently, a tick strain resistant to all available classes of pesticides was reported in Brazil (Reck et al., 2014). Early detection of acaricide resistance is fundamental to the establishment of sustainable control strategies, mainly related to the choice of acaricide. Diagnostic methods based on molecular strategies are fast, highly sensitive and specific compared to bioassays (Pruett et al., 2002), as a single individual can be used to identify several mutant alleles. The diagnosis is based on the detection of resistance when there are low frequencies of alleles that confer resistance in a given population (Rosario-Cruz et al., 2005). Fipronil is a phenylpyrazolic insecticide used to control R. microplus infestations in cattle. Its mode of action is attributed to the blocking of the gamma-Aminobutyric acid gated chloride ion-channel (GABA-Cl; Cole et al., 1993; Durham et al., 2001; Bloomquist, 2003). The resistance of R. microplus to fipronil was first described in Uruguay and developed relatively quickly (approximately 10 years after its initial marketing) compared to resistance to other acaricides (Castro-Janer et al., 2009), such as organophosphates and pyrethroids (12–16 years; Cardozo and Franchi, 1994). This rapid development of resistance could be explained by cross-resistance determined by metabolic detoxification, as described in the cattle tick resistance against organophosphates (Miller et al., 1999), or by target site insensitivity due to mutations selected by others chemical groups with the same mode of action, as described in the cross-resistance between pyrethroids and DDT (Nolan et al., 1977; Schnitzerling et al., 1983). The cross-resistance between organochlorine cyclodiene derivatives and phenylpyrazoles insecticides has been demonstrated in several studies with insects (Colliot et al., 1992; Cole et al., 1993, 1995; Bloomquist, 1994; Scott and Wen, 1997; Brooke et al., 2000) and recently, in R. microplus field strains from Brazil and Uruguay (Castro-Janer et al., 2015).